Understanding the neural systems specifically driving compulsive drug seeking is essential for developing treatments for addiction as a disease. The orexin/hypocretin (ORX) system in the lateral hypothalamic area (LHA) has been widely implicated in motivation for natural and drug rewards, in addition to its role in arousal and other physiological functions. This system may play a critical role in the extremely high levels of motivation and craving underlying addiction, but it is unknown what contribution specific populations of ORX neurons play in compulsive drug seeking. Different ORX neurons may perform different functions based on anatomical location or connectivity. For example, it has been hypothesized that lateral-LHA ORX neurons play a greater role in reward seeking, whereas medial-LHA ORX neurons are more associated with arousal and stress. Identification of specific populations that selectively regulate compulsive drug seeking will allow the development of treatments for drug craving that do not affect other ORX-mediated functions such as arousal or normal hedonic states. Direct investigation of the contributions of specific ORX neuron populations has been limited, however, by an inability to target individual ORX neurons or subsets of neurons for physiological recording or manipulation, particularly in rat models, where there are robust models of compulsive drug seeking and addiction. Here we will investigate the roles of individual ORX neurons in rat models of compulsive drug and natural reward (sucrose) seeking. We will use transgenic rats that express Cre recombinase selectively in ORX neurons. We will direct optogenetic and chemogenetic constructs to these neurons to selectively modulate ORX neuron activity and behavior. We will use multi-neuron ensemble recording combined with optogenetic tagging to characterize the activity of ORX neurons while animals are seeking cocaine or sucrose. We will also selectively modulate either lateral or medial hypothalamic subpopulations of ORX neurons during reward seeking using selectively-targeted designer receptors exclusively activated by designer drugs (DREADDs). The studies will focus on regulated and compulsive models of drug and sucrose seeking to examine changes in ORX function as compulsive drug/reward seeking develops. We hypothesize that lateral ORX neurons will encode and regulate drug/reward seeking early in regulated seeking, but that medial regions will also be recruited in compulsive seeking, particularly in cocaine seeking. These studies are the first in a series designed to investigate which networks of ORX neurons are specifically involved in drug addiction and compulsive reward seeking behaviors. The two aims to be investigated in this initial project are: 1. Characterize lateral and medial ORX neuronal activity in early and late cocaine and sucrose seeking using behavioral electrophysiology and optogenetic tagging of ORX neurons. 2 Selectively enhance or suppress lateral and medial ORX neuronal activity in early and late cocaine and sucrose seeking using DREADDs to selectively excite or inhibit lateral or medial ORX neuron ensembles.